1. Field of the Invention
This invention relates to a current transformer that detects a sine-wave alternating current and a half-sine-wave rectified current within a desired level of accuracy, and a static type electric energy meter in which a sine-wave alternating current and a half-sine-wave rectified current are input, and incorporated with such a current transformer.
2. Background of the Invention
Electric energy meters are used to calculate the electric power consumption of electric machines and instruments in homes and industries. Both electromechanical type electric energy meters and static type electric energy meters are commonly used. Static type electric energy meters, however, have come into wide use with the advancement of technology concerning electronics. In a static type electric energy meter, it is necessary to detect both an alternating load voltage value and an alternating load current value.
A current transformer for use in a static type electric energy meter includes a magnetic core provided with one turn of primary winding, through which an alternating load current is passed, and hundreds turns or thousands turns of secondary winding, to which a detecting resistor is connected in parallel. The resistor represents the resistance of the secondary winding. The measuring accuracy in a current transformer is evaluated by the amplitude error F(I) and the phase error φ. Accordingly, the values should be lower than a determined value.
The amplitude error F(I) is determined by the following equation:
                                          F            ⁡                          (              I              )                                =                                    -                                                                    R                    Cu                                    +                                      R                    b                                                                    ω                  ⁢                                                                          ⁢                                      L                    s                                                                        ⁢            sin            ⁢                                                  ⁢            δ                          ,        where                            (        1        )                RCu is the resistance of the resistor, which represents the resistance of the secondary winding (Ω),    Rb is the resistance of the detecting resistor (Ω),    δ is the loss angle of magnetic core (rad),    ω=2πf is the angular frequency (rad/s), and    Ls is the inductance of the secondary winding (H).
The inductance Ls of the secondary winding is determined by the following equation:
                                          L            s                    =                                                                      μ                  r                                ⁢                                  μ                  0                                ⁢                                  A                  e                                ⁢                                  N                  s                  2                                                            l                e                                      ⁢                                                  ⁢                          (              H              )                                      ,        where                            (        2        )                μr is the incremental relative permeability of the magnetic core,    μ0 is the permeability of a vacuum, 4π×10−7 (H/m),    Ae is the effective cross section of the magnetic core (m2),    Ns is the number of turns of the secondary winding, and    le is the mean magnetic path length of the magnetic core (m).
From equation (1) and equation (2) shown above, the following equation (3) can be derived:
                              F          ⁡                      (            I            )                          =                              -                                                            (                                                            R                      Cu                                        +                                          R                      b                                                        )                                ⁢                                  l                  e                                                            ω                ⁢                                                                  ⁢                                  μ                  r                                ⁢                                  μ                  0                                ⁢                                  N                  s                  2                                ⁢                                  A                  e                                                              ⁢          sin          ⁢                                          ⁢                      δ            .                                              (        3        )            
The phase error φ is determined by the following equation:
                    ϕ        =                                            tan                              -                1                                      ⁡                          (                                                                                          R                      Cu                                        +                                          R                      b                                                                            ω                    ⁢                                                                                  ⁢                                          L                      s                                                                      ⁢                cos                ⁢                                                                  ⁢                δ                            )                                ⁢                                          ⁢                                    (              rad              )                        .                                              (        4        )            
Using equation (2) and equation (4), the following equation can be obtained:
                    ϕ        =                                            tan                              -                1                                      ⁡                          (                                                                                          (                                                                        R                          Cu                                                +                                                  R                          b                                                                    )                                        ⁢                                          l                      e                                                                                                  ω                      ⁢                                                                                          ⁢                                              μ                        r                                            ⁢                                              μ                        0                                            ⁢                                              N                        s                        2                                            ⁢                                              A                        e                                                              ⁢                                                                                                                ⁢                cos                ⁢                                                                  ⁢                δ                            )                                ⁢                                          ⁢                                    (              rad              )                        .                                              (        5        )            
As shown by equations (3) and (5), it is effective to use a magnetic core provided with a high value of incremental relative permeability, μr, in order to reduce the amplitude error F(I) and the phase error φ. From this perspective, it is advantageous to use a magnetic core made of 80% nickel permalloy or a magnetic core made of nano-crystalline alloy provided with an incremental relative permeability of more than 10,000, in order to produce excellent current transformers. These current transformers can be widely used in a static type electric energy meters for industrial use according to the standard IEC 62053-22. As the load current value is great as 100 Arms or more in industrial instruments, only a small amount of completely zero-symmetrical sine-wave alternating current reduced by an external special transformer is input into such a static type electric energy meter.
A current transformer incorporating a magnetic core made of 80% nickel permalloy or nano-crystalline alloy having an incremental relative permeability of more than 10,000, however, is not suitable for a static type electric energy meter into which a load current is directly input, for domestic use, or for relatively small loaded industrial use. In these uses, a zero-asymmetrical alternating load current including a direct current component, which is caused by half-wave rectifier circuits or phase controlling circuits used in current electronic devices or instruments, is directly input into the electric energy meter. A current transformer incorporating a magnetic core material having a high incremental relative permeability is designed to accept only zero-symmetrical sine-wave alternating current. When a zero-asymmetrical alternating input current is input into the primary winding of such a current transformer, which contains a direct current component, the input of the zero-asymmetrical alternating input current causes a saturation of the magnetic core and falsifies the current detection.
The measuring of the accuracy range and the testing method to measure a half-sine-wave rectified current are set forth in the representative standard IEC 62053-21, 8.2 and Annex/A concerning the static type electric energy meter, into which a load current is directly input, for domestic use and relatively small loaded industrial use. The standard IEC 62053-21 requires for the static type electric energy meter, which has a performance specification to measure a sine-wave alternating current having a maximum effective current value Imax(Arms) within the range of accuracy defined by this IEC standard, to measure a half-sine-wave rectified current having an effective current value Imax·20.5 (Arms). Thus, a wave height 20.5·Imax(Aop) falls within the range of accuracy defined by this IEC standard. As a half-sine-wave rectified current contains a direct current component having a value of 1/π times the wave height, the value of the direct current component in a half-wave rectified waveform current having an effective current value Imax·20.5 (Arms) or a wave height 20.5·Imax(Aop) is (20.5/π)·Imax(ADC). A current transformer incorporating a Co-based amorphous core, which satisfies the desired accuracy for an alternating current measurement in a static type electric energy meter, even if the current containing such a big value of direct current component is input into the primary winding, is proposed in U.S. Pat. No. 6,563,411B1. Also, current transformers incorporating toroidal cores made of the Co-based amorphous alloy VITROVAC® (Vacuumschmelze GmbH & Co. KG, Hanau, Germany) 6030F or 6150F are presented by Vacuumschmelze GmbH & Co. KG (hereafter abridged as VAC).
The core proposed in U.S. Pat. No. 6,563,411 B1, or made of Co-based alloys VITROVAC® 6030F or 6150F, presented by VAC, when used for a static type electric energy meter for residential use or relatively small loaded industrial use, satisfies the desired accuracy for electric energy measurement in these static type electric energy meters when a sine-wave alternating current and a half-sine-wave rectified current, respectively, are input. However, when using the core proposed in U.S. Pat. No. 6,563,411 B1, VITROVAC® 6030F, or 6150F, the size of the core and current transformer required to achieve the desired electric specification for these static type electric energy meters is too large.
For example, six kinds of current transformers for a static type electric energy meter are shown in Table 2 of “Current Transformers for Electronic Watthour Meters” (http://www.vacuumschmelze.de/dynamic//docroot/medialib/documents/broschueren/kbbrosch/Pb-cteng.pdf). The five kinds of current transformers have a rated maximum effective current value Imax(Arms) of 20 Arms, 40 Arms, 60 Arms, 100 Arms, and 120 Arms, respectively, with respect to a sine-wave alternating current according to the standard IEC 61036, which is comparable to the new standard IEC 62053-21. As shown in Table 2, the five kinds of current transformers each have a rated maximum peak value 20.5·Imax(Aop) of 36 Aop, 72 Aop, 80 Aop, 113 Aop, and 158 Aop, respectively, with respect to a half-sine-wave rectified current as shown in the same table.
The specifications of the magnetic cores used in these current transformers are set forth in “Cores for Current Transformers for Electronic Energy Meter” (http://www.vacuumschmeize.de/dynamic//en/home/products/coresampinductivecomponents/applications/cores/coresforcurrenttransformersforelectronicenergymeter.php). Based on these specifications, it is understood that a toroidal core made of the Co-based amorphous alloy VITROVAC® 6030F is used in the current transformer having a rated maximum effective current value Imax(Arms) of 20 Arms with respect to the sine-wave alternating current, and the toroidal cores made of Co-based amorphous alloy VITROVAC® 6150F are used in other current transformers.
According to the IEC standard, current transformers having rated maximum effective current values Imax(Arms) of 20 Arms, 40 Arms, 60 Arms, 100 Arms, and 120 Arms, respectively, with respect to the sine-wave alternating current should have rated maximum peak values 20.5·Imax(Aop) of 28.3 Aop, 56.6 Aop, 84.9 Aop, 141 Aop and 170 Aop, respectively, with respect to a half-sine-wave rectified current. Thus, strictly speaking, the current transformers having rated maximum effective current values Imax(Arms) of 60 Arms, 100 Arms, and 120 Arms with respect to the sine-wave alternating current are not appropriate for application in a static type electric energy meter, as determined by the rated maximum effective current value Imax(Arms) and designed under the standard IEC 62053-21. Only the current transformers having rated maximum effective current values Imax(Arms) of 20 Arms and 40 Arms, respectively, with respect to a sine-wave alternating current may be used for a static type electric energy meter determined by the rated maximum effective current value Imax(Arms) and designed under the standard IEC 62053-21.
For example, in order to apply a current transformer selected from those described above for a static type electric energy meter having a rated maximum effective current value Imax(Arms) of 60 Arms with respect to the sine-wave alternating current defined in the IEC standard, the current transformer having a rated maximum effective current value Imax(Arms) of 100 Arms with respect to the sine-wave alternating current must be used. Additionally, none of the current transformers described above for a static type electric energy meter having a rated maximum effective current value Imax(Arms) of 120 Arms determined by the IEC standard with respect to the sine-wave alternating current as determined by the IEC standard can be selected. Moreover, the current transformers described above have rated maximum effective current values Imax(Arms) of 20 Arms and 40 Arms, respectively, with respect to the sine-wave alternating current is excess performance, because they have maximum peak values of 20.5·Imax(Aop) of 36 Aop and 72 Aop, respectively, concerning a half-sine-wave rectified current. According to the IEC standard, however, these values should be 28.3 Aop and 56.6 Aop, respectively. Thus, size of these current transformers should be reduced in comparison to the sizes described.
As mentioned above, it is desirable to present electromagnetic specifications and ways of design of magnetic cores and current transformers well fit for the IEC standard IEC62053-21 for static type electric energy meters. Although U.S. Pat. No. 6,563,411B1 explains some qualitative requirements for a current transformer and a magnetic core to be incorporated in a static type electric energy meter defined under the IEC (IEC 1036, which is the same as IEC 61036, the old standard corresponding to IEC 62053-21), it is not enough to present quantitative guidelines to design a magnetic core and a current transformer to be applied for a static type electric energy meter determined by the IEC standard IEC 1036.